Ultrasound (US) imaging has provided useful information about the interior characteristics of an object or subject under examination. An ultrasound imaging system has included an ultrasound probe that houses a transducer array including one or more transducer elements. FIG. 1 shows an example of a single transducer element 102 with a width “w” and an acceptance angle θ at an imaging depth of “z”.
In B-mode imaging, the transducer array transmits a radio-frequency pulse of wavelength λ into a scan field of view. As the pulse traverses the medium of the object or subject, portions of the pulse are attenuated, scattered, and/or reflected off structure boundaries present in the medium. Some of the reflections (echoes) traverse back to the transducer elements 102, being received thereby. The echoes correspond to an axial slice through the object or subject and are processed to generate scan lines, which are used to produce a scan plane, or a two dimensional image of the slice.
The transducer in FIG. 1 is not focused and the wave close to the transducer surface is planar. The wave starts to diverge at depth zT, where the distance can be roughly estimated as shown in Equation 1:
                                          Z            T                    ~                      1            4                          ⁢                                            w              2                        λ                    .                                    Equation        ⁢                                  ⁢        1            The depth beyond zT is generally referred to as the far-field and the wave starts to diverge. For a rectangular aperture, the angle of divergence in the far field θ referred also as acceptance angle can be roughly estimated as shown in Equation 2:
                                          sin            ⁡                          (                              θ                2                            )                                ~                      λ            w                          .                            Equation        ⁢                                  ⁢        2            
B-mode imaging has been combined with synthetic aperture imaging. With synthetic aperture imaging, the transducer element 102 translates along a scan path and transmits pulses at predetermined different locations on the scan path and the received echoes from the different transmissions are combined, for example, the signals are delayed and summed in phase, to produce an image. This is shown in FIG. 2 in which the transducer element 102 translates between first and second locations 104 and 106. The synthesized aperture length, “L,” directly depends on the acceptance angle θ and the imaging depth “z” as shown in Equation 3:
                    L        =                  2          ⁢          z          ⁢                                          ⁢          tan          ⁢                                    θ              2                        .                                              Equation        ⁢                                  ⁢        3            
The beam pattern of the synthesized array using monostatic synthetic aperture focusing can be approximated at the focus as described in S. Nikolov, Synthetic Aperture Tissue and Flow Ultrasound Imaging, Ph.D. dissertation, 2001, Technical University of Denmark and as shown in:
                                          sin            ⁡                          [                                                                    2                    ⁢                    π                                    λ                                ⁢                L                ⁢                                                                  ⁢                sin                ⁢                                                                  ⁢                θ                            ]                                                                          2                ⁢                π                            λ                        ⁢            L            ⁢                                                  ⁢            sin            ⁢                                                  ⁢            θ                          .                            Equation        ⁢                                  ⁢        4            
Using small angle approximation (sin θ≈tan θ≈θ, valid for angles below 5 degrees), it can be shown that the 3 dB beamwidth can be approximated as shown in Equation 5:
                                          b                          3              ⁢                                                          ⁢              d              ⁢                                                          ⁢              B                                =                      C            ⁢                                                  ⁢            λ            ⁢                          z              L                                      ,                            Equation        ⁢                                  ⁢        5            where C is a constant that generally depends on the weights applied to the signals prior to summation. As a results, a larger “L” results in a narrower beam which gives better resolution.
From Equation 3 and Equation 5, the larger the acceptance angle θ, the better the resolution. The energy transmitted into the tissue is proportional to the transducer area, so that a larger “w” results in a better signal to noise ratio. Unfortunately, a larger “w” corresponds to a smaller acceptance angle θ, and, thus, poorer resolution.